Molecular signals identified that let some organisms regrow lost heads

Manipulating the signals can produce a worm with a head at both ends.

Although they're nothing exciting to look at, Planaria are remarkable creatures in at least one regard: when cut in half, they're able to regenerate two complete animals. The tail half will regrow a replacement head, while the head will extend a new tail (you can also cut them in half along their main body axis and each side will grow a new mirror image of itself).

Although this would seem to provide a huge evolutionary advantage—the animal can bounce back after being partly eaten—a number of related species lack this ability. Some have it in a more limited form, where they can regrow a tail but not a head. Now, scientists have used these other species to start to understand what happens when an organism regenerates missing body parts.

Initially, all regeneration goes through a similar process, whether it's the formation of a new limb by amphibians or a new head by a planarian. Once the wound heals over, a group of cells called a "blastema" forms at its surface. These are unspecialized stem cells that divide and grow and gradually spin off all the specialized cells (muscles, nerves) that need to be replaced. As the regeneration is completed, the blastema slowly dissipates, leaving behind replacement tissues.

In a series of papers that track the limited regeneration found in some species of planarians, the authors first confirmed that all of the initial steps of wound-healing and blastema formation occur normally in these animals. So if there's a problem with remaking a head, it happens a bit later in the process.

The authors of the different papers took different methods to get there (ranging from high-throughput DNA sequencing to making some educated guesses), but all of them came to the same conclusion: when regeneration doesn't happen, it's because the blastema isn't getting the right signals to tell it what kind of tissue it needs to form. There's one signal (Wnt/ß-catenin) that tells the cells that they should form a new tail and another (tyrosine kinase signaling) that indicates a new head is needed. One of the papers notes that this finding supports a model proposed over a century ago by geneticist and Nobel Prize winner Thomas Hunt Morgan, who suggested planarians relied on a combination of "head stuff" and "tail stuff" to regrow the right tissues.

A lack of regeneration can occur from some combination of not seeing enough of the right signal or seeing too much of the wrong one. In some cases, researchers were able to manipulate the signaling networks in order to override the normal behavior. For example, a head could be made to regenerate a second head, facing in the opposite direction, provided the right molecular tools were used—and this in a species that didn't normally regenerate heads at all. Animals with two tails were also created.

None of this gets in to why the organisms in question don't regenerate their heads in the first place. But it can help us to start answering the question. The signaling networks identified here are involved in a lot of important biological processes, and activating them could easily have unintended consequences. Now that we know what they are, we can start looking at what those consequences might be and how the planarians that can regenerate avoid these consequences.

So how fast is the process? You'd think the animal would die of starvation before a new head is grown. Also, does the new head enable the animal to function normally? I guess with animals of such low intelligence they act more on instinct but still, fascinating.

So how fast is the process? You'd think the animal would die of starvation before a new head is grown. Also, does the new head enable the animal to function normally? I guess with animals of such low intelligence they act more on instinct but still, fascinating.

That may be why the ability was lost in other species, functionally it didn't actually increase their survivability.

So how fast is the process? You'd think the animal would die of starvation before a new head is grown. Also, does the new head enable the animal to function normally? I guess with animals of such low intelligence they act more on instinct but still, fascinating.

It's not terribly long. Some of the species actually reproduce by dividing, so starvation is likely not a major issue.

It's not mentioned in the article, but if you split the head end lengthwise, they can grow to have two heads. You can even split the entire organism lengthwise and wind up with two organisms.

So how fast is the process? You'd think the animal would die of starvation before a new head is grown. Also, does the new head enable the animal to function normally? I guess with animals of such low intelligence they act more on instinct but still, fascinating.

It's not terribly long. Some of the species actually reproduce by dividing, so starvation is likely not a major issue.

It's not mentioned in the article, but if you split the head end lengthwise, they can grow to have two heads. You can even split the entire organism lengthwise and wind up with two organisms.

Article wrote:

(you can also cut them in half along their main body axis and each side will grow a new mirror image of itself)

To determine if it needs a head or a tail, it uses at least two chemicals. Are both used when it is split in half or is a third chemical used to trigger a half-body. As you segment the body up into more sections (head,tail, leg, arm, etc) it would seem by this method the combination of unique chemicals grows quite large. As the number of chemical trigger signals increase so does the complexity of that feedback system, and the likelyhood for the wrong trigger to occur. Example: Lose your hand and it tries to grow a head as a replacement due to the trigger chemicals being wrong somehow.

A warning to the squeamish: this tale may make you squirm,As a scientist plays Fruit Ninja - only with a tiny worm.He sliced it and he diced it, for he was a little bored,(And to act out manly fantasies, with his little ninja sword).But Wilberforce the worm-like one just took this in his stride,He grew a front, he grew a back, he grew a new left side.The scientist was soon found dead with worms covering his face;Just another victory for the Planarian Master Race.

Seems we already have the ability to regenerate but for some reason our default is to simply heal not regeneration. I wonder what the evolutionary advantage is to simple healing vs regeneration.

I think I read once, I don't remember where, or if this is exactly correct, that scar tissue "heals" a wound much faster than regeneration, which leaves the animal more vulnerable for a longer period of time. So it was a historical trade-off that made sense for our isolated ancestors -- better to limp away from predators than get eaten while regenerating. This trade-off no longer makes sense for us as social, apex predators.

Seems we already have the ability to regenerate but for some reason our default is to simply heal not regeneration. I wonder what the evolutionary advantage is to simple healing vs regeneration.

I'm a bit skeptical of the source. That said, we have another term for uncontrolled growth. It's called cancer. It would not surprise me if the ability to regenerate, which removes some limits to growth beyond the current extent of the body, is also a risk factor for cancer.